The preparation of nanoporous materials is a high-ranking goal of current research. The goal is to develop methods which on the one hand allow a cost-effective and straightforward access to such materials and on the other hand include the possibility of implementing said method on an industrial scale.
There are several methods for preparing nanoporous materials which, however, have not been implemented on a larger scale due to the complex preparation and costly use of materials and machines. Hence, for example, silicon-based aerogels having open-cell structures between 1 and 30 nm could be synthesised according to Kistler by a sol-gel process. Due to a subsequent supercritical drying, this initially very simple sol-gel process becomes a complicated and tedious endeavour. Here, the solvent must be extracted carefully from a prepared silica gel by supercritical drying without damaging the extremely fine nanostructure (S. S. Kistler. Nature, 127:1 (1931)).
Based on Colton's and Suh's idea to saturate polymers with foaming agents and subsequently expand them above their glass temperature, Krause et al. were able to add supercritical CO2 to polyether imides and polyether sulfones and thus prepare nanoporous materials they called polymer nanofoams (J. S. Colton; N. P. Suh. Polymer Engineering and Science, 27:485-492 (1987); J. S. Colton; N. P. Suh. Polymer Engineering and Science, 27:493-499 (1987); J. S. Colton; N. P. Suh. Polymer Engineering and Science, 27:500-503 (1987); D. F. Baldwin et al. Polymer Engineering and Science, 36:1437-1445 (1996); D. F. Baldwin et al. Polymer Engineering and Science, 36:1446-1453 (1996)). Here, the preparation includes the saturation of a polymer film having a thickness of several millimeters at pressures of approx. 50 bar and temperatures from 100-250° C. for several hours (B. Krause et al. Macromolecules, 35:1738-1745 (2002); B. Krause et al. Macromolecules, 34:8792-8801 (2001)).
Merlet et al. developed self-foaming polymer systems and achieved pore diameters in the range of 10 to 700 nm. Using the polymer polyphenylquinoxaline that contains heat-labile tert-butyloxycarbonyl groups, they were able to release CO2 and isobutene in the polymer by increasing the temperature and thus obtain a nanoporous material (S. Merlet et al. Macromolecules, 41:4205-4215 (2008)).
JP S58-67 423 A describes the manufacture of a polycarbonate foam.
CN 1318580 describes the manufacture of a polymer foam.
U.S. Pat. No. 7,838,108 B2 has already claimed nanocellular polymer foams. This patent describes a method for manufacturing the protected material on the basis of a typical solution process of a foaming agent in a polymer. As described in the above references, this method involves very high pressures, temperatures and extremely long residence times of the polymer in the foaming agent. Thus, also this method is uneconomical and reasonably applicable only to small sample thicknesses. Moreover, a thermal conductivity from 1 to 10 mW/(m·K) is indicated for the protected polymer material in claim 1 of U.S. Pat. No. 7,838,108 B2, which lies in a range that can only be achieved by evacuating porous materials to date (vacuum insulation panels).
EP 2 185 620 B1 describes a method for preparing micro- and nanoporous xerogels based on polyurea. Generally, in the preparation of a xerogel or also an aerogel, a gel consisting of a solid and a solvent is prepared from a sol. The gel transforms into a porous material by exchanging the solvent by a gas (air, for example). Here, it must be remembered that the gel body is already the maximum volume that can be obtained after drying the porous material. Thus, the expected density of the porous material only depends on the ratio of solid to solvent. There is therefore no volume increase by an expanding foaming agent in terms of a foaming process. In addition to the inorganic silica-based aerogels described by Kistler, the mentioned patent describes a method that also makes organic aerogels/xerogels accessible.
EP 1 646 687 B1 describes the manufacture of nano- or microporous syndiotactic polystyrene. Here, a gel is prepared from a suitable solvent and syndiotactic polystyrene and subsequently processed into a porous syndiotactic polystyrene material by super- or nearcritical drying with CO2. As described in this patent, during the super-/nearcritical drying (residual solvent content <1 wt % (percent by weight)) the solvent is removed nearly completely and the expansion to normal pressure is carried out during 60 to 120 minutes. The examples of the patent describe that after drying the polymer material has the same shape and dimensions as the original gel. Thus, also this method does not provide an increase in volume in terms of an expansion of a foaming agent to produce a porous polymer material but an exchange of solvent by a gas (air). The ratio of polymer to solvent in the starting gel is significantly responsible for the resulting density of the dried porous polymer material. Therefore, already during the gel manufacturing process care is taken to keep the polymer content in proportion to the solvent as small as possible (1-50 wt %) to obtain a target material with the lowest possible density.
All mentioned methods are either very time-consuming or expensive.
Hence, there is a substantial interest in simplifying the existing methods for preparing polymer foams and therefore also in improving the obtained polymer foams, which allows to reduce productions costs and to open up new areas of application.